1. Field of the Invention
The present invention relates to a centrifugal compressor, and an impeller and an operating method of the same, particularly blade geometry of the impeller.
2. Description of the Related Art
A centrifugal compressor that compresses fluid using a rotary impeller has been widely used in a variety of plants in the related art. Recently, it has been required to enlarge the operating range for a stable operation of the impeller, due to the increased concerns in the lifecycle cost, and problems relating to energy and the environment.
The operating range for a stable operation of the impeller is determined by a surge that makes periodic change in pressure or flow rate due to increase of a recirculation area that is generated by flow separation when flow rate decreases more at a small flow rate side, and choke that does not increase any more at a large flow rate side.
The blade geometry of the impeller of the centrifugal compressor that has a large effect on the operating range, for example, as disclosed in JPA-2002-21784, is constructed on the basis of a blade angle distribution from the inlet to the outlet of a flow channel of the impeller. Therefore, the blade angle distribution is determined in consideration of both manufacturability and aerodynamic performance.
The blade angle distribution is generally determined to satisfy target specifications, such as efficiency, pressure ratio, and operating range using flow analysis or design tool, for each operation. However, in this determination, the relationship between an appropriate operating range and the blade angle distribution is not known. Accordingly, it was difficult to determine whether the operating range could be increased or not by adjusting the blade angle distribution.
As described above, since the relationship between an appropriate operating range and the blade angle distribution is not known, when the operating range for the target specifications in insufficient, in order to compensate for the insufficiency, the operating range is enlarged by adjusting the main dimensions, such as longitudinal length and diameter of the inlet of the impeller, or by applying casing treatment for increasing the operating range of the small flow rate side.
However, the main dimensions, such as longitudinal length and the diameter of the inlet of the impeller, had a larger effect on the rotor vibration as compared with the blade angle distribution, such that it was required to re-examine the design of the rotor vibration to adjust the main dimensions. Accordingly, examination items were increased, which reduced the efficiency in the design. Further, since additional process of applying the casing treatment was required to increase the operating range for the small flow rate side, manufacturing cost is increased and efficiency of performance is correspondingly decreased.